Make PRODUCT_CHARACTERISTICS a single-valued variable

Values should be comma-separated, and things won't go down well
if there are multiple words in this variable.

Bug: 123783710
Test: Put this in product_config.mk and ran multiproduct_kati:
      ifneq (1,$(words $(PRODUCT_CHARACTERISTICS)))
        $(error oops)
      endif
Change-Id: I5627f416093d68d8d19916a914739686b74d392f
diff --git a/core/product.mk b/core/product.mk
index 1a566f7..838673c 100644
--- a/core/product.mk
+++ b/core/product.mk
@@ -162,7 +162,9 @@
 # used for adding properties to build.prop of product partition
 _product_list_vars += PRODUCT_PRODUCT_SERVICES_PROPERTIES
 _product_list_vars += PRODUCT_ODM_PROPERTIES
-_product_list_vars += PRODUCT_CHARACTERISTICS
+
+# The characteristics of the product, which among other things is passed to aapt
+_product_single_value_vars += PRODUCT_CHARACTERISTICS
 
 # A list of words like <source path>:<destination path>[:<owner>].
 # The file at the source path should be copied to the destination path